With the diversification and development of network applications, the bearing rate of main lines of an optical transmission network increases from 10 gigabits per second (Gb/s) and 40 Gb/s to 100 Gb/s. The 100 Gb/s optical transmission systems will become a mainstream of the next-generation optical network.
Currently, the optical transmission system with a line rate of 100 Gb/s is generally realized through a polarization multiplexing system. The polarization multiplexing system modulates two data source signals of 56 Gb/s to two optical signals, which have mutually orthogonal states of polarization, respectively at a sending end, and a polarization beam combiner (PBC) combines the two optical signals into one polarization-multiplexed optical signal. The polarization-multiplexed optical signal carries data of 112 Gb/s, and is sent to an optical fiber link for transmission.
A receiving end of the polarization multiplexing system uses a polarization controller (PC) to control the state of polarization (SOP) of the received polarization-multiplexed optical signal, and compensate for and rectify the loss and change of the SOP introduced on the optical fiber link. A polarized light separating apparatus such as a polarization beam splitter (PBS) or a polarization detector obtains single-SOP polarized light. The system that uses an optical device to implement polarization demultiplexing at the receiving end is called an optical transmission system for optical polarization division multiplexing.
FIG. 1 is a schematic structural diagram of an optical transmission system for optical polarization division multiplexing in the prior art. The system is a 112 Gb/s optical transmission system implemented through Differential Quadrature Phase Shift Keying (DQPSK) modulation, polarization multiplexing, and optical polarization division multiplexing.
The input data sources in FIG. 1 are four 28 Gb/s electric signals: D1, D2, D3, and D4. At a sending end, every two data signals (electric signals) are modulated onto the optical signal through a DQPSK modulator, where D1 and D2 are modulated onto an optical signal Y, and D3 and D4 are modulated onto an optical signal X. The optical signal Y and optical signal X are generated after a light source of the same laser is split by a coupler. The optical signal Y and optical signal X are both single-SOP optical signals, and SOPs thereof are mutually orthogonal, as shown in FIG. 2 and FIG. 3. In this way, the optical signal Y and optical signal X bear 56 Gb/s data respectively. The optical signal Y and optical signal X are combined by a PBC into a polarization-multiplexed optical signal, which is sent onto an optical fiber link for transmission. The SOPs of the optical signal Y and optical signal X before and after the combination are as shown in FIG. 3. The polarization-multiplexed optical signal obtained through combination bears 112 Gb/s data.
Upon arriving at the receiving end, the polarization-multiplexed optical signal is processed by a feedback control loop including a PC, a PBS, an optical splitter, a feedback quantity extracting module, and a search track module. The PC rotates the SOP, and the PBS outputs two pieces of single-SOP polarized light. In this way, the optical polarization division multiplexing is realized. The feedback quantity extracting module includes a radio frequency (RF) detecting module or a pilot detecting module. That is, the RF is used as a feedback quantity to control the PC to adjust the SOP, or the pilot is used as a feedback quantity to control the PC to adjust the SOP. When the pilot is used as a feedback quantity, the polarization multiplexing sending end needs to add a controllable attenuator or modulator to scramble the optical signal X or optical signal Y.
A change of SOP of the polarization-multiplexed optical signal is as shown in FIG. 4. Two SOPs of the optical signal at the sending end are mutually orthogonal. When the optical signal is transmitted through an optical fiber link, because of various types of loss and interference on the link, the two SOPs of the optical signal arriving at the receiving end are not orthogonal any more. The receiving end outputs a control quantity according to a feedback signal found and tracked, and uses the output control quantity to control the PC to adjust the SOP of the received polarization-multiplexed optical signal. The feedback signal extracted by the feedback quantity extracting module may be pilot information added into the optical signal Y or crosstalk power of the optical signal X in the optical signal Y. The search track module in the feedback loop controls, on a basis of a maximum feedback quantity or a minimum feedback quantity, the PC in real time to make the feedback quantity maximum or minimum. That is, the search track module makes the PC rotate the whole polarization-multiplexed optical signal until the optical signal X is deflected to a horizontal direction or the optical signal Y is deflected to a vertical direction. It can be seen from FIG. 4 that, when the PC adjusts the SOP until the optical signal X is deflected to the horizontal direction, the PBS filters out an optical signal X′ of the horizontal SOP and an optical signal Y′ of the vertical SOP so that the optical polarization division multiplexing is realized. The optical signal X′ is superimposition of the optical signal X and a component of the optical signal Y in the horizontal direction, and the optical signal Y′ is a component of the optical signal Y in the vertical direction. Conversely, when the PC adjusts the SOP until the optical signal Y is deflected to the vertical direction, an optical signal Y′ is superimposition of the optical signal Y and a component of the optical signal X in the vertical direction, and an optical signal X′ is a component of the optical signal X in the horizontal direction.
In the process of implementing the present invention, the inventor finds at least the following defects in the prior art: The optical signal output from the same output port of the PBS is not fixed, and may be an optical signal X′ or an optical signal Y; the separated optical signal X′ includes the component of the optical signal Y in the vertical direction, namely, the output optical signal X′ includes the component of the optical signal Y; or the separated optical signal Y′ includes the component of the optical signal X. Crosstalk is noticeable and affects the performance of the optical transmission system.